1. Field of the Invention
The present invention relates to a combined cycle electric power plant and more particularly to improved coordinated steam load distribution control in a combined cycle plant.
2. State of the Prior Art
In the design of modern electric power plants, it is a significant object to achieve the greatest efficiency possible in the generation of electricity. To this end, steam generators are designed to extract heat efficiently from and to use the extracted heat to convert a fluid such as water into superheated steam at a relatively high pressure. Further, such steam generators have been incorporated into combined cycle electric generating plants including both gas and steam turbines wherein the exhaust gases of the gas turbine are used to heat water into steam then to be transferred to the steam turbine. Typically, steam generators include a water heating section or economizer tube, a high pressure evaporator tube and finally a superheater tube, whereby water is gradually heated while increasing levels of pressure are applied thereto to provide from the superheater tube, superheated steam to supply the steam turbine. A condenser is associated with the steam turbine to receive the spent steam therefrom and for converting it into water condensate to be fed back to the steam generator.
In a combined cycle electric power plant, the steam turbine is combined with a gas turbine whereby the heated exhaust gases of the gas turbine, otherwise lost to the atmosphere, are used to heat the circulated fluid and to convert it into steam to drive the steam turbine. In this manner, a significant reduction in the fuel required to heat the steam is achieved and the heat contained in the gas turbine exhaust gases is effectively utilized. Further, an afterburner associated with the exhaust exit of the gas turbine serves to additionally heat the gas turbine exhaust gases, whereby the heat required to generate sufficient steam to meet load requirements is provided. In particular, under conditions of relatively high loads, wherein the heat of the gas turbine exhaust gases is insufficient to supply the steam requirements, the afterburner is turned on to further heat the gas turbine exhaust gases.
In combined cycle operation, there is a particular need to coordinate the control of the separate turbines, as well as the control of the afterburners. It is desired that the steam turbine be operated in what is called a "turbine following" mode whenever the plant is supplying electrical power to a load, such that the steam turbine follows the gas turbines, with each afterburner positively following a respective gas turbine. In this mode, the steam produced by the gases exhausted by the afterburners is used in total by the steam turbine. In distributing load among the operating turbines, and determining the load change rates for the respective turbines when responding to changed plant demand, there must be coordinated control so as to optimize efficiency and response time. Further, there is required a control system which can automatically determine which turbines and/or afterburners are in condition for coordinated control, or have been selected for coordinated control, and proceed with any combination thereof in coordinated control while other elements are simultaneously under a lower level of control. Desirably, the coordinated control flexibility is available through startup, synchronization, and the full range of plant loading.
In the loading of the steam turbine of a combined cycle plant, it is desirable that the steam turbine be automatically loaded subject only to the overall plant load demand, i.e., that the distributive share of the plant load carried by the steam turbine be automatically controlled as the steam turbine follows the gas turbines. In such automatic loading, or unloading, of the steam turbine, the load change rate should be subject only to monitored turbine operating conditions, e.g., steam throttle pressure.
Reference is made to the above-listed copending applications dealing with combined cycle power plants, and particularly to the description of the prior art set forth therein under the heading "Background Of The Invention". The referenced applications, and in particular Ser. No. 399,790, describe the basic system concept of coordinated plant operation, and in particular disclose a specific analog embodiment of a coordinated system. The advantages offered by coordinated control, namely (1) better response and therefore better availability; (2) better efficiency and therefore lower cost of operation; and (3) more organized and better response to contingencies and therefore better reliability, are better achieved by incorporation of overall digital control at all levels of operation. With overall digital control, additional and improved control functions and loops can be incorporated, and the ability to switch such functions in and out of operation, i.e., system flexibility, can be vastly increased.
Reference is further made to Westinghouse Descriptive Bulletin 23-830, dated September, 1972, and entitled "PACE Automation and Control System", which bulletin is also incorporated herein by reference. The descriptive bulletin summarizes the PACE System Control, including the plant operating modes and control panel layouts.